1. Field of the Invention
The invention relates to an arbitration device and method, more particularly to an arbitration device and method with a dynamic priority mechanism.
2. Description of the Related Art
With the advancement of imaging information systems, a blend of image layers, such as picture in picture (PIP), on screen display (OSD), caption, cursor, background image, etc., is often presented on a screen. For a real-time playback system, prior to display on a screen, it is necessary to retrieve and suitably process data of all image layers to be presented on the screen before they can be blended in sequence. Since the data of each image layer is normally large, the system is built with large-capacity registers for storing the data of the image layers, which is not economical. In a more economical approach, data of an image layer to be presented is processed immediately upon acquisition for real-time display. Therefore, during the process of playback, data retrieval, processing and display are parallel-processed. However, in such a parallel-processing scheme, while data of each image layer must be retrieved for processing, use of a shared data bus necessitates the grant of only one data access request associated with one of the image layers at a time. As a result, an arbitration issue with regard to data access requests among the image layers arises.
At present, there are a number of arbitration schemes known in the art, such as a fixed-priority scheme, a round-robin scheme, a weighted round-robin scheme, etc. Nevertheless, when faced with a wide variety of display applications, any specific arbitration scheme is not suited for application as a universally optimal design. For instance, on a screen with video images and text captions, the position of the text captions may overlie the video images (in this case, the text captions must is be retrieved earlier than the video images), or the position of the text captions may underlie the video images (in this case, the text captions must be retrieved later than the video images).
The aforementioned conventional arbitration schemes have their inherent disadvantages. For example, when data of an image layer that should be retrieved earlier (such as data to be presented on an overlying position of a screen) is actually retrieved later than data of other image layers (such as data to be presented on an underlying position of the screen), the data of the other image layers will be stored in a register for an undesirably longer period of time. As a result, it is needed to configure larger-capacity registers in the system to ensure sufficient data amounts for real-time playback. Inability to retrieve required data immediately has an adverse affect on real-time playback. Moreover, the longer the time data is required to be stored in a register, the larger will be the required capacity of the register. It is noted that each image layer is associated with a corresponding register. Therefore, when the number of image layers is large, the required number of registers will be large as well.
It is apparent from the foregoing that the conventional arbitration schemes require large-capacity buffers to minimize any adverse influence on real-time screen display and to ensure playback continuity. Consequently, circuit size and costs are increased when the conventional arbitration schemes are in use, which is not very cost-effective.